1. Field of the Invention
The present invention relates to a fluid coupling having a quick acceleration and deceleration mechanism.
2. Prior Art
Conventional fluid couplings include a fixed charge type coupling in which the circuit is filled with a fluid (hydraulic oil) at all times during operation, and a variable charge type coupling in which the amount of fluid (hydraulic oil) in the circuit is variable to allow a change in transfer torque capacity. Variable charge type fluid couplings include a variable speed type and an oil charge-discharge type.
FIG. 10(a) is a schematic representation of the above-described variable speed type fluid coupling. The coupling comprises an impeller (pump impeller) 102 connected to a driving shaft 101, and a runner (turbine impeller) 103 connected to a driven shaft 104 in opposing relation to the impeller 102 to form a fluid circuit. Hydraulic oil in the circuit circulates through an oil tank 105, an oil pump 106 and an oil cooler 107, and it can be partly replaced (increased or decreased) through a scoop tube 109 that is controlled through an actuator 108.
This type of fluid coupling functions for the purpose of controlling the number of revolutions (i.e., rotational speed) of the driven side and of no-load starting of an electric motor on the driving side and enables a lowering in the running cost and also a lowering in the driving machine cost.
FIG. 10(b) shows schematically the above-described oil charge-discharge type fluid coupling, in which hydraulic oil in the circuit circulates through an oil charge-discharge switching valve 111 connected to the outlet of the oil cooler 107, and oil is charged through a replenishment pipe 112 extending into the circuit and discharged through a return pipe 113 extending into the oil tank 105. The oil in the circuit is constantly returned to the oil tank 105 at a predetermined flow rate through a nozzle 114.
This type of fluid coupling functions to cut off power (clutch action), absorb torsional vibration, effect no load starting of a prime mover and reduce starting resistance, and enables individual operation of an engine, on-off operation of a driven machine, easy starting and acceleration of a prime mover, etc.
FIG. 10(c) shows schematically the above-described fixed speed (fixed charge type) fluid coupling, which is designed to operate with the circuit filled with a fluid at all times. This type of fluid coupling functions for the purpose of reducing the starting resistance, lessening and absorbing vibration and impact and absorbing torsional vibration and also functions as a torque limiter, and it functions to protect an electric motor and machine connected thereto.
The conventional variable speed type fluid coupling (shown in FIG. 10(a)) enables a desired rotational speed to be set over a wide controllable range by varying the position of the scoop tube 109 and is therefore suitable for a multi-point operation. However, since the moving of the scoop tube 109 is relatively slow, the acceleration response speed, that is, the time needed to increase the rotational speed from its lowest to highest, of this type of fluid coupling is relatively slow, e.g. about 10 seconds at its fastest.
The oil charge-discharge type fluid coupling (shown in FIG. 10(b)) allows the rotational speed to be changed over between two points, i.e., the highest rotational speed and the lowest rotational speed, by opening and closing the oil charge-discharge switching valve 111 and is therefore suitable for sole operation of a driving machine and an on-off operation of a driven machine. However, since the hydraulic oil is discharged from the nozzle 114, provided at the outer periphery of the hydraulic oil chamber, at all times during the operation of the driven machine, it leads to a lowering in the efficiency in the case of a small-sized fluid coupling. In addition, since the amount of hydraulic oil discharged from the nozzle 114 is limited, the acceleration-deceleration response speed is slow. Therefore, this type of fluid coupling cannot be applied to uses where a fast acceleration-deceleration response speed is required.
The fixed speed fluid coupling (shown in FIG. 10(c)) is suitable mainly for absorbing and reducing impact force but is unable to control the rotational speed.
Accordingly, if a fluid coupling having any of the conventional structures is applied to an intermittently operating apparatus, for example, a descaling pump used to remove scales from the surface of a steel material being manufactured in an iron works, there is a great practical difficulty involved because response to the speed change is slow. For this reason, it has heretofore been conventional practice to rotate the driven shaft continuously at its highest rotational speed during a no-load operation and to throttle the flow of hydraulic oil using a valve or the like.